Spacecraft that generate and utilize large quantities of power require heat management systems to absorb heat from the heat generating sources, transport the heat over some distance to a waste heat radiator and then radiate the heat into space. Currently, such systems utilize pumped liquid for cooling and heat transport. Such systems, however, suffer from high electrical power consumption typically amounting to approximately 10% of the thermal power transported, and a substantial temperature change in the liquid is required to absorb the heat which thus lowers the average temperature of the liquid at the waste heat radiator. This in turn necessitates a larger waste heat radiator. Thus, such systems, while effective for their intended purpose, have substantial drawbacks when employed in spacecraft, namely, relatively high power consumption in an environment where power generating capability is necessarily restricted due to weight concerns, and the need for large radiators in an environment where both size and weight are of substantial concern.
To overcome these difficulties, there has been proposed a two phase heat management system which dissipates heat from the heat load by evaporating a liquid and rejects heat by condensing the vapor. Such a system can transport heat with no substantial temperature drop between the head load and the waste heat radiator. Much less pumping power is required in such a system because the mass flow rate of liquid/vapor is substantially less than for a pumped liquid system. This is due to the fact that the heat of vaporization of the liquid is large in comparison to the relatively small sensible heat transferred to a liquid stream with a reasonable temperature differential. The lesser mass flow rate and the higher temperature differential between condensing vapor and sink temperature obtainable in such a system minimize radiator size.
In one such system, evaporators are placed in heat exchange relation with the heat load and are provided with a heat exchange fluid in the liquid phase by conventional centrifugal pumps. Vapor or a liquid/vapor mixture emanating from the evaporators is in part fed through a radiator and in part bypassed about the radiator to be merged again at a mixing valve downstream from the radiator. The degree of mix and flow is controlled by setting a desired temperature for the system and the output of the mixing valve, after the two streams are thoroughly mixed, is inputted to a constant pressure accumulator which in turn is connected to the inlet of the centrifugal pump. In the typical case, the fluid in the accumulator is maintained slightly below saturation at a temperature and pressure consistent with temperature control requirements of the heat load. If the liquid is not in a slightly subcooled state at the pump input, cavitation may occur resulting in system failure. For when the pump cavitates, it will provide no fluid to the evaporators with the consequence that they will cease generating vapor. The radiator heat load will then likewise vanish for with no vapor to condense, the radiator will tend to fall in temperature toward sink temperature. This will result in a lower vapor pressure in the evaporators and in the vapor space in the radiator. This in turn will cause the accumulator to expel liquid which will flow backwardly through the mixer to fill the vapor space in the radiator until pressure equilibrium is reestablished in the system at a lower pressure level. The lowering of the pressure level at the accumulator results in less subcooling of the liquid in the accumulator which in turn increases the tendency of the pump to cavitate. It is considered that control errors of as little as 3% at the mixing valve can bring about a situation where cavitation is prone to occur.
Such a system also lacks adequate turn down capability, that is, the ability to rapidly respond to considerable reductions in the heat load at the evaporator. In spacecraft, it is highly desirable to maintain the heat sources or loads, frequently electronic gear, at a tightly controlled constant temperature and in a two phase heat management system, system pressure is determinative of the temperature at which the liquid evaporates in the evaporators responsible for maintaining the heat load at the desired temperature.
Also of concern is the heat load at the radiator. As is well known, the sink temperature to which the radiator is exposed varies greatly dependent upon the relationship of the spacecraft relative to, for example, the sun and the earth. When a spacecraft is orbiting the earth and is between the earth and the sun, a typical design temperature for the sink temperature would be approximately 385.degree. R.
Conversely, when the earth is interposed between the orbiting spacecraft and the sun, the sink temperature may descend to as low as 150.degree. R. or lower.
In the situation where the evaporator heat load decreases and sink temperature is decreased, a system such as described previously will tend to condense and greatly subcool the fraction of the flow passing from the evaporator to the mixing valve, the degree being dependent on wink temperature. The bypass fraction intended to mix with the subcooled fraction from the radiator and warm it to a temperature slightly below the saturation temperature of the heat exchange fluid being employed may not accomplish the same, either because of lesser sensible heat in the bypass fraction due to decreased heat load or because of insufficient sensible heat because of the greatly reduced temperature of the condensate due to decreased sink temperature, or both. The subcooled temperature of the heat exchange fluid passing through the condenser is, of course, limited by the freezing temperature of the fluid employed and in the situation described, fluid freeze up can occur resulting in system failure. Thus such a system cannot reliably cope with the shedding of heat load or large fluctuation in sink temperature.
Analysis of a system such as described, where ammonia is employed will show that such a system has virtually no turndown capability for the varying sink temperature encountered by spacecrft, that is, that the minimum heat load for which the system will remain operative during the low sink temperature exposure is almost equal to the design heat load.
In a two phase system as described, a fixed mass of gas fills a volume on one side of a bellows in the accumulator while the heat exchange fluid fills the other side. As pressure level in the system decreases, the pressure of the fixed gas mass in the accumulator urges the bellows to expel liquid on its opposite side into the system. The volume of gas required to drive the bellows is, of course, related to the allowable change in system pressure level necessary to accomplish the desired expulsion.
Because, as mentioned previously, evaporating temperature, and thus temperature control of the heat load, is dependent upon system pressure, where close control is required, system pressure changes must be held to a minimum. Thus, the volume of the fixed mass of gas in the accumulator must be quite large in comparison to the volume of liquid that can be expelled from the accumulator in order to maintain substantially constant system pressure. Consequently, to maintain tight temperature control, the accumulator in such systems must be made quite large and the resulting size and weight are highly undesirable in spacecraft. Conversely, if the size and weight of the accumulator is decreased, sensitive and accurate temperature control of precision components such as electronics is degraded, which is likewise undesirable.
The present invention is directed to overcoming one or more of the above problems.